Conventionally, a wheel bearing apparatus that supports a wheel of a vehicle that freely rotationally supports a wheel hub mounts a wheel via a rolling bearing. The wheel bearing apparatus includes those for a driving wheel and those for a driven wheel. Considering the structure of the apparatus, in general, the inner ring rotation type is used for the driving wheel and both the inner ring rotation type and the outer ring rotation type are used for the driven wheel type. There are four wheel bearing apparatus generation types. The first generation type includes a wheel bearing with a double row angular contact ball bearing, etc., fit between a knuckle, forming a part of a suspension apparatus, and a wheel hub. The second generation type includes a body mounting flange or a wheel mounting flange directly formed on the outer circumference of an outer member. The third generation type includes one inner raceway surface directly formed on the outer circumference of the wheel hub. The fourth generation type includes inner raceway surfaces formed on the outer circumferences, respectively, of the wheel hub and an outer joint member.
Usually, a wheel is rotatably supported by a double row rolling bearing relative to a suspension apparatus. However, a wheel bearing apparatus provided with a double row tapered roller bearing is used for heavy duty vehicles such as off-road cars, trucks etc.
In the inner ring rotation type of wheel bearing apparatus, it has been adopted, as a type to axially secure the inner ring onto the wheel hub. It is a so-called self-retaining structure where the inner ring is secured on the wheel hub by caulking the end of the wheel hub. FIG. 7 shows a representative example of this structure. It is called a second generation type. It includes a wheel hub 51 and a wheel bearing 52 that is fit onto the wheel hub 51.
The wheel hub 51 has an integrally formed wheel mounting flange 53 on its outer circumference on one end. A cylindrical portion 51b axially extends, through a shoulder portion 51a, from the wheel mounting flange 53. The wheel hub Si also has a torque transmitting serration 51c on its inner circumference. Hub bolts 53a are arranged equidistantly along the periphery of the wheel mounting flange 53.
This wheel bearing 52 includes an outer member 54 integrally formed with a body mounting flange 54b on its outer circumference. The body mounting flange 54b is mounted on a knuckle (not shown). The outer member 54 inner circumference includes double row outer raceway surfaces 54a, 54a. A pair of inner rings 55, 55, each formed on its outer circumference with tapered inner raceway surfaces 55a, is arranged opposite to one of the double row outer raceway surfaces 54a, 54a. Double row tapered rollers 57, 57 are freely rollably contained between the outer and inner raceway surfaces, via cages 56. A larger flange portion 55b, for guiding the tapered roller 57, is formed on the larger diameter side of the inner raceway surface 55a of each inner ring 55. A smaller flange portion 55c, to prevent fallout of the tapered roller 57, is on the smaller diameter side of each inner ring 55. The pair of the inner rings 55, 55 are arranged with their smaller flange portions 55c abutting against each other to form a tapered roller bearing of the back-to-back duplex type.
The wheel bearing 52 is press fit onto the cylindrical portion 51b, via a predetermined interference. The larger end face 55d of the outer side inner ring 55 abuts against a shoulder portion 51a of the wheel hub 51. The wheel bearing is also axially secured relative to the wheel hub 51 by a caulked portion 58. The caulked portion 58 is formed by plastically deforming the end of the cylindrical portion 51b radially outward. Seals 59, 59 are mounted in annular openings formed between the outer member 54 and the pair of inner rings 55, 55. The seals 59, 59 prevent leakage of lubricating grease sealed within the bearing and the entry of rain water or dust into the bearing from the outside.
The wheel hub 51 is hardened by high frequency induction hardening in a region from the shoulder portion 51a, forming a base of the wheel mounting flange 53, to the cylindrical portion 51b. It has a hardened layer 60 with a surface hardness of 50-64 HRC. The caulked portion 58 remains “as is” so that it still has a surface hardness after forging.
The region of the hardened layer 60 is set as shown in FIG. 8. A position P of its inner side end is positioned within a range from an edge P0 of a chamfered portion 55e of the inner ring 55 to a position P1, which corresponds to the width “a” of the larger flange portion 55b (i.e. a root portion of the larger flange portion 55b). This makes it possible not only to improve the durability of the wheel hub 51, due to reduction of fretting wear at the fit surfaces of the inner rings 55, 55, but to reduce an amount of expansion of the end of the cylindrical portion 51b of the wheel hub 51. Thus, this suppresses the deformation of the inner raceway surface 55a and the larger flange portion 55b of the inner ring 55 caused accompanied with the caulking. Accordingly, it is possible to obtain a smooth guidance of the tapered rollers 57 by reducing the contacting surface pressure between the rollers 57 and the inner rings 55 and thus to improve the durability of the inner rings 55.
In addition, a radius of curvature “r” of the chamfered portion 55e of the inner ring 55 is set within a range of R1.0-R2.5 mm. This makes it possible to prevent stress concentrations from being caused at the root portion of the caulked portion 58. Additionally, it prevents an excessive hoop stress from being caused on the outer circumference of the inner ring 55 due to an increased amount of expansion of the cylindrical portion 51b by caulking. Thus, it is also possible to improve the strength and durability of the inner ring 55.
In addition, an annular recessed portion 61 is formed on the end of the cylindrical portion 51b of the wheel hub 51. This recessed portion 61 is formed so that it is smaller than a depth “b” (5 mm) from the larger end face (inner side end face) of the inner ring 55. This makes it possible to assure a predetermined inner ring securing force while maintaining the strength and rigidity of the wheel hub 51. Also, it makes the plastic deformation easy. Thus, it is possible to suppress the hoop stress caused in the inner ring 55. According to the wheel bearing apparatus of the prior art, the durability of the wheel hub 51 can be improved due to a reduction of fretting wear at the fit surfaces of the inner rings 55. In addition, it is possible to reduce an amount of expansion of the end of the cylindrical portion 51b of the wheel hub 51. Thus, this suppresses the deformation of the inner raceway surface 55a and the larger flange portion 55b of the inner ring 55 caused by the accompanying caulking (e.g. see Japanese Laid-open Patent Publication No. 202184/2007).
On the other hand, in a wheel bearing apparatus used for the part-time 4-WD (4 wheel drive) that can perform switching between the 2-WD (2 wheel drive) and the 4-WD, there is a wheel bearing apparatus that includes a rolling bearing, such as a deep groove ball bearing, within the inner circumference of a wheel hub for freely rotatably supporting a driving shaft within the bearing (e.g. see Japanese Laid-open Patent Publication No. 271044/2007
The problem does not arise in the conventional wheel bearing apparatus where the retaining ring groove is located toward the wheel mounting flange side rather than the center of the rolling element. However, the problem arises in the wheel bearing apparatus shown in, for example, FIG. 9, where a rolling bearing 62 is fit onto the inner circumference of a wheel hub 64. That is, in such a wheel bearing apparatus, a retaining ring 63 for positioning and securing the rolling bearing 62, is mounted on the wheel hub 64 at a position near the caulked portion 58. A drive shaft D/S is inserted into the center of the wheel hub. The driving shaft D/S is rotatably supported by the rolling bearing 62 relative to the wheel hub 64 and can be connected to the wheel hub 64, via a clutch mechanism C/S.
As shown in FIG. 10, the end of the cylindrical portion 64a of the wheel hub 64 is initially formed as a straight cylindrical body 65. It is caulked using a swingable caulking tool 66. In such a case, it is believed that micro cracks 68 would be caused to start from a retaining ring groove 67 formed on the inner circumferential surface of the cylindrical portion 64. The cracks 68 occur in positions that are difficult to find in a visual inspection and may sometimes grow to be large ones when the cracks are subjected to large moment loads during running of a vehicle. This results in the induction of a capital deficiency due to the fallout of the caulking portion 58. Thus, it is necessary to have careful attention during machining of the retaining ring groove 67 and the inspection after machining. Accordingly, there are problems that much time is required for machining of the wheel hub 64 and forming of the caulked portion. Thus, the working efficiency is reduced as well as the manufacturing cost is increased.